(1) Field of the Invention
This invention relates to a rubber composition containing at least 20% by weight of styrene-butadiene copolymer and having excellent rebound resilience and processability.
(2) Description of the Prior Art
Heretofore, polybutadiene rubber has not quite been used alone, but has been used together with natural rubber, polyisoprene rubber, styrene-butadiene copolymer and the like.
As the polybutadiene rubber, there have mainly been used high cis-polybutadienes obtained by using a Ziegler catalyst, and low cis-polybutadienes obtained by using an organolithium compound. However, although these polybutadienes have an excellent rebound resilience as compared with other rubber materials owing to the low glass transition temperature, they contain a large amount of a low molecular weight component which is poor in the rebound resilience because the molecular weight distribution thereof is set wider so as to increase the processability and prevent the cold flow of the raw rubber.
On the other hand, there are known polybutadienes having a narrow range of molecular weight distribution, which are obtained by using an organolithium compound, and polybutadienes having a bimodal molecular weight distribution, but they are inferior in the wear resistance and tensile strength to the above polybutadienes having a wider range of the molecular weight distribution.